Anatomy

Electronics Repair Manuals

Schematic Diagrams and Service Manuals

Get Instant Access

Although this review focuses on the olfactory system, it is necessary to mention, at least briefly, other sensory channels involved in chemosensory perception. All the senses can be stimulated by chemicals, which in fact, typically activate not only one but several of the ''chemical senses''. For example, nicotine not only activates the olfactory nerves, but also produces activation of the intranasal chemosensory trigeminal system.

Main Olfactory System

Olfactory perception starts at the level of the olfactory epithelium in the roof of the nasal cavity. Olfactory receptor neurons (ORN) are embedded within the respiratory epithelium and send their axons through the cribriform plate towards the olfactory bulbs. ORN carry olfactory receptors (OR) which are the key to olfactory information processing (see below). In the olfactory bulb ORN axons synapse with second order neurons, the mitral cells. The wiring between the olfactory epithelium and the olfactory bulb is characterized by a convergence of ORN axons. Specifically, all ORN carrying the same OR converge in the same site within the bulb, called ''glomerulus''. Axons from the mitral cells follow the olfactory tract and divide into two bundles. Most fibers directly project to the pyriform and entorhinal cortices as well as to the amygdalae (all structures formerly subsumed under the term ''limbic system'') whereas a minority of fibers project through the thalamus towards the orbito-frontal cortex [1].

Compared to other sensory modalities the olfactory system has some particularities. First, the majority of the olfactory fibers do not cross but project ipsilaterally in the brain. Second, most olfactory fibers bypass the thalamus and project very rapidly and directly in the pyriform cortex, amygdalae, and entorhinal cortex which are implicated in emotional and memory processing [2]. This difference in central anatomy has been claimed to be partly responsible for the emotional load olfactory memories can carry [3]. In contrast to other sensory modalities, no main olfactory cortex has yet been found. Numerous works indicate the orbitofrontal cortices to be an important relay in olfactory information processing [4].

Trigeminal System

The trigeminal system provides the somato-sensory innervation to the nasal mucosa. Somato-sensory input from the skin, the nasal and oral cavities, respectively, is mediated by the trigeminal system. Since most odorous compounds stimulate trigeminal nerve endings, at least at higher concentrations, this system is almost always co-activated in the perception of odors. With few exceptions almost all odorants have been shown to exhibit trigeminal activation to some extent [5] (e.g., mint has a somewhat fruity odor, but also evokes a typical cooling effect which is mainly trigemi-nally mediated). The main modalities supplied by the trigeminal system are temperature, pain, touch, and irritation. Testing the chemosensory intranasal trigeminal system psychophysically is more complex than olfactory testing. Since olfactory thresholds are always lower than the trigemi-nal thresholds for a given substance, olfactory biases are obvious.

The olfactory system is unable to localize the site of stimulation when one nostril receives clean air and the other nostril simultaneously receives an odor at the same time. In contrast, trigeminal stimulation can be localized. This difference is used to solve the bias inherent to trigeminal testing. Using lateralisation paradigms, trigeminal function can be easily and rapidly measured in a clinical context [6, 7]. Since the olfactory and trigeminal systems are so closely related anatomically and physiologically, there is a strong interaction between the two systems [8, 9]. In patients with olfactory loss, the trigeminal function is also weakened [10, 11]. Older literature on trigeminal trans-sections also discusses its impact on olfactory

Fig. 1. Schematic diagram illustrating the axonal connectivity pattern between the nose and the MOB. The OE in mice is divided into four zones (zones I through IV) that are defined by the expression of odorant receptors. Olfactory sensory neurons in a given zone of the epithelium project to glomeruli located in a corresponding zone (zones I through IV) of the MOB. Axons of sensory neurons expressing the same odorant receptor (red or dark blue) converge to only a few defined glomeruli. NC Neocortex; AOB accessory olfactory bulb. Reprinted (abstracted/excerpted) with permission from Mori K, Nagao H, Yoshihara Y (1999). The olfactory bulb: coding and processing of odor molecule information. Science 286 (5440): 711-715. Copyright (1999) AAAS

Fig. 1. Schematic diagram illustrating the axonal connectivity pattern between the nose and the MOB. The OE in mice is divided into four zones (zones I through IV) that are defined by the expression of odorant receptors. Olfactory sensory neurons in a given zone of the epithelium project to glomeruli located in a corresponding zone (zones I through IV) of the MOB. Axons of sensory neurons expressing the same odorant receptor (red or dark blue) converge to only a few defined glomeruli. NC Neocortex; AOB accessory olfactory bulb. Reprinted (abstracted/excerpted) with permission from Mori K, Nagao H, Yoshihara Y (1999). The olfactory bulb: coding and processing of odor molecule information. Science 286 (5440): 711-715. Copyright (1999) AAAS

function [12]. However, patients with complete trigeminal loss are extremely rare and no large study has so far been conducted to investigate the effects of trigeminal loss on olfactory function.

Gustatory System

The gustatory system provides the five basic tastes; sweet, sour, salty, bitter, and umami (glutamate). The latter, which resembles mainly the taste of chicken soup, has long been claimed in the Asian literature to be a basic taste quality [13, 14], whereas the western scientific community considered umami mainly as a ''taste enhancer''. This controversy was resolved when monosodium glutamate receptors were found on the tongue surface acting as specific taste receptors [15]. Molecular biological knowledge about taste receptors started to emerge a few years ago. Most basic taste qualities are not mediated by just one receptor type; several receptors act, for example, as sweet receptors. Many other taste modalities have been postulated (metallic taste, fat taste) and are currently under investigation. Future research is expected to clarify the coding mechanisms in taste perception.

Taste receptors are located within the taste buds, which are situated on all papillae except the filiform type. The highest densities of taste buds are found on the tongue and palate but they are also found throughout the entire oral cavity, hypopharynx and subglottic larynx [16]. Like ORNs, gustatory sensory receptor cells have the ability to regenerate [17]. Neural supply for these cells is provided by the facial, glossopharyngeal, and vagal nerves. The facial nerve innervates the anterior two thirds of the tongue, while the glossopharyngeal and vagal nerves innervate the posterior third of the tongue, as well as the hypopharynx and larynx. Like olfaction, taste fibers project ipsilaterally into the basal ganglia and brain stem. All gustatory fibers (facial, glossopharyngeal and vagus) innervating the oral-pharyngeal cavity converge into the nucleus solitarius within the brain stem.

Vomeronasal System

In 1813, Jacobson described a mucosal organ located on each side of the nasal septum and which was subsequently named ''Jacobson's organ'' [18, 19]. Later, the vomeronasal organ (VNO, consisting of vomeronasal epithelium, nerve, and accessory olfactory bulb) was shown to mediate effects attributed to pheromones [20]. A pheromone is a chemical molecule or compound which is secreted by one member of a species and, as soon as it is perceived by another member of the same species, elicits physiological, behavioral, or endocrinological effects [21, 22]. According to the original definition of Karlson and Luscher [20] such effects must be species-specific. While the functionality and biological relevance in most animals is well documented, there is ongoing debate about the functional significance of the vomeronasal pouch in humans. Some authors claim to find neuronal activity within the vomeronasal epithelium [23] while many other studies suggest that the vomeronasal duct is nonfunctional in humans, with some vomeronasal nerves missing and lack of accessory olfactory bulbs in adults [24-27]. Furthermore, a vomeronasal duct is not always present in humans; recent investigations revealed that approximately 60% of humans have one [25]. Nevertheless, a few papers indicate that pheromonal-like effects occur in humans [28, 29] and several vomeronasal-like receptor genes have been found in the human genome, one of which is expressed in the olfactory epithelium (V1r) [30]. It is not yet known whether these receptors are functional or not. Their expression, however, indicates that putative ''human pheromones'' may act via the main olfactory system.

Was this article helpful?

0 0
Acupuncture For Cynics

Acupuncture For Cynics

Have You Always Been Curious About Acupuncture, But Were Never Quite Sure Where To Stick The Needles? If you associate acupuncture with needles, pain and weird alternative medicine then you are horribly misinformed about the benefits of the world's oldest form of medicinal treatment.

Get My Free Ebook


Post a comment